Thermal evaporation sources for wide-area deposition

ABSTRACT

A thermal evaporation sources are described. These thermal evaporation sources include a crucible configured to contain a volume of evaporant and a vapor space above the evaporant.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 15/645,927, filed Jul. 10, 2017, which is a continuation ofU.S. patent application Ser. No. 14/625,433, filed Feb. 18, 2015, whichis a continuation of U.S. patent application Ser. No. 12/250,172, filedOct. 13, 2008, which claims priority benefit of U.S. Provisional Pat.Appln. No. 60/998,640, filed Oct. 12, 2007, each of which isincorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of Contract No.ADJ-1-30630-12, awarded by the National Renewable Energy Laboratory.

BACKGROUND OF THE INVENTION

The high-vacuum deposition of thin films, such as Cu(InGa)Se₂, bythermal evaporation onto horizontally-oriented substrates which arespatially situated above the evaporation source (herein referred to as“vertical evaporation”) is well known, and may be useful for formingabsorber layers for photovoltaic devices. Generally speaking, avertically-evaporating thermal evaporation source comprises asubstantially closed vessel containing an evaporant, typically in liquidbut possibly in solid form, with at least one effusion nozzle tunnelingthrough the upper surface of the vessel through which the elementalvapor effuses. The relative simplicity of the effusion source design isone of the significant advantages of vertical evaporation.

However, a problem with vertical (i.e., upward) evaporation is that thesubstrate, in particular a rigid substrate, may only be supported at itsedges to avoid either shadowing the substrate surface from deposition,or marring the substrate surface by physical contacting. The restrictionof supporting the substrate at its edges can for some substrates limitthe substrate temperature during deposition. One particular example isglass and more particularly soda-lime glass, where using an excessivelyhigh substrate temperature (such as in the vicinity of the softeningpoint in the case of glass) can cause warpage or breakage of thesubstrate. This limiting of the substrate temperature may ultimatelylimit the desired properties of the deposited film, such as thephotovoltaic conversion efficiency of Cu(InGa)Se₂ absorber layers onsoda-lime glass, as it is well known that the photovoltaic conversionefficiency of solar cells utilizing Cu(InGa)Se₂ absorber layerstypically increases monotonically with substrate temperature up to atemperature of approximately 550° C.

The most basic requirements of a thermal evaporation source are a volumecomprising the elemental source material, and single or plural effusionnozzles to direct the elemental vapor, generated by the melt surface,from the source interior to the substrate. In the case of avertically-evaporating source, the effusion nozzles will ideally bewithin close proximity to and axially oriented normal to the meltsurface. In the simplest designs, the effusion nozzles will be aimedvertically and located directly above the melt surface, as illustratedin FIG. 1 . FIG. 1 shows a prior art vertical evaporating source 30 witheffusion nozzles 36 passing through heat shielding 22 and situateddirectly above and in close proximity to the surface of evaporantmaterial 14. The substrate to be coated is indicated at 26. The devicealso comprises an evaporation chamber 18 and a containment box 12.

It is desirable to minimize the external surface area of the source inorder to minimize the thermal load. Further, it is desirable to minimizethe aspect ratio of the source (the ratio of the major dimension to theminor dimension, upon viewing the surface of the evaporant, so as tomaximize temperature uniformity within the source. A non-uniform melttemperature results in variations in vapor pressure above the melt,causing variations in effusion rate through the nozzles, ultimatelycontributing to non-uniform film thicknesses on the substrate. Furtherhindering uniform deposition is the fact that the temperature profile ofthe source may be expected to change as depletion of the elementalsource material occurs, thereby further reducing thermal conductancealong the major axis and reducing deposition uniformity. A potentialremedy to the problems exhibited by the configuration in FIG. 1 is theconfiguration described by Baron et al (U.S. Pat. No. 4,401,052), inwhich a separate low-aspect-ratio melt chamber is heated to generate avapor of the evaporant from a substantially isothermal evaporantsurface. This vapor is then directed into a manifold and out throughmultiple effusion nozzles to the substrate. A problem with thisconfiguration is that in the case of evaporants which require very hightemperatures for sufficient vapor generation, the large surface area ofthis configuration may result in an unacceptably high thermal loading.Furthermore, the actual physical fabrication of this design ischallenging.

SUMMARY OF THE INVENTION

The invention provides a thermal evaporation source that includes:

a crucible configured to contain a volume of evaporant and a vapor spaceabove the evaporant;

a manifold body having within it a hollow expansion chamber that isflowably connected to the vapor space via one or more restrictionorifices;

one or more effusion nozzles flowably connected to the expansion chamberand exiting an outer surface of the thermal evaporation source, thenozzle(s) oriented to direct an evaporant vapor flow out of the sourcevertically downward, in one or more horizontal directions, or in one ormore directions intermediate between horizontal and vertically downward;and

a heater capable of heating some or all of the thermal evaporationsource to a temperature sufficient to produce the one or more evaporantvapor flows when a vacuum is applied to the thermal evaporation source.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view of a prior art verticalevaporating source.

FIG. 2 is a cross-sectional side view of a sideways-evaporating effusionsource according to the invention.

FIGS. 3A and 3B are cross-sectional side views of a single-nozzledownwards-evaporating sources according to the invention.

FIG. 4 is a cross-sectional side view of a multiple-nozzle,downwards-evaporating source according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides evaporation sources for high-vacuumdeposition onto linearly-translating, wide-area substrates, where“wide-area” refers to the requirement of multiple effusion point sourcesto achieve uniform deposition across the width of the substrate as thesubstrate moves relative to the evaporation source, with the widthwisedirection being defined as perpendicular to the direction oftranslation. The devices and methods of this invention may be applied tovacuum thin film deposition in general, with one particularly usefulapplication being the formation of chalcopyrite based thin films forphotovoltaic applications.

Compared with other designs, the disclosed configurations offerexcellent control, higher areal throughput, and relative insensitivityto debris from accumulated unutilized evaporant materials. In someembodiments, they allow the use of higher substrate temperatures, whichin turn allows more efficient operation of the resultant photovoltaicmodules. Specific configurations disclosed also allow improvedcontrollability, doubled areal throughput, or mitigation of dislodgeddebris that may disturb the process.

Compared with vertical evaporation, when considering downwards- orsideways-evaporating sources, the fundamental source configurationchanges in that the nozzle axis cannot be situated above the meltsurface and aligned normally to it.

The invention discloses a series of configurations fordownwards-evaporating and sideways-evaporating sources. Substratesituations intermediate between that of “downwards evaporation” and“sideways evaporation” may also be used, according to the invention.

Sideways-Evaporating Source

FIG. 2 shows a cross-sectional side view of one possible configurationof a sideways-evaporating effusion source 130 capable of simultaneouslydepositing on two wide-area substrates 126, according to the invention.Viewed from the top, the cross-section of the source 130 may be circularor square. In the case where the cross-section is circular, the source130 may comprise a threaded lid and body (not shown) for ease ofcharging evaporant 114.

The device includes a volume of evaporant 114 contained in a crucible146, which also contains a vapor space 118 above the evaporant. Vaporfrom vapor space 118 enters an expansion chamber 150 enclosed within amanifold body 140, with the restriction orifice so that the net vaporflow into the manifold is upwards and normal to the evaporant 114surface. Optionally, an internal restriction orifice 142 may be formedin a crucible lid 142, the function of which is to cause the evaporant114 vapor pressure in the expansion chamber to be less than thethermodynamic saturation pressure of the evaporant 114, therebyinhibiting evaporant 114 condensation within the manifold body or in theeffusion nozzles 136.

A heating element 144 (typically unitary) heats some or all of theassembly. There may be a single heating element, or two or more may beused. For example, one element may primarily heat the manifold whileanother primarily heats the crucible 146 containing evaporant 114volume. Or, one element may heat the crucible and evaporant whilemultiple elements heat the manifold. Heating element 144 may be eitherspiral-wound around the perimeter of the source 130, or serpentine withthe straight runs of the heating element oriented vertically.Alternatively a number of single, straight heating elements, orientedvertically, may be disposed around the perimeter of the source 130.Other configurations are also acceptable according to the invention. Oneexemplary heating element material known in the art is graphite, and ifnecessary it may be electrically insulated from the body of theevaporation source with a high-temperature electrical insulator such asboron nitride.

One or more effusion nozzles 136 are situated on a single face of themanifold body, or on two opposite faces (if the source 130 has a squarecross-section) or diametrically opposed (If circular) as in FIG. 2 . Inthe case of multiple effusion nozzles 136 per face (e.g., three on eachof 2 faces as shown in FIG. 2 ), the effusion nozzles may progressivelyincrease in size (not shown in FIG. 2 ) with distance from the manifoldentrance, in order to compensate for the vapor pressure drop along thelength of the manifold. In some embodiments, the effusion nozzles 136are of the conical nozzle design disclosed in U.S. Pat. Nos. 6,982,005and 6,562,405, the entire disclosures of which are incorporated hereinby reference. An effusion plume 148 of evaporant vapor exits theeffusion nozzles and is deposited on the substrate(s) 126.

During operation, the manifold is maintained at a temperature above thesaturation point of the vaporized evaporant 114 within the manifold toprevent condensation. An insulating layer 138 typically surroundssubstantially the entire assembly with the exception of the effusionnozzle(s) 136 and electrical feed-throughs for the heating element(s)144.

The evaporant may be any element, or compound thereof, that is known inthe art for high-vacuum deposition in forming a photovoltaic absorberlayer. Nonlimiting examples include copper, indium gallium and selenium.In use, substrate 126 translation direction is normal to the plane ofthe page.

Generally speaking, in order to minimize the surface-area-to-volumeratio of the source 130, the source 130 should be cylindrical ingeometry; i.e., circular in cross-section when viewed from the top. Thecrucible 146 may have a larger circumference than the manifold, asprolonged operating duration may be achieved by increasing the volume ofthe evaporant 114 chamber. Simultaneously, the circumference of themanifold should be limited in order to reduce the thermal loading of thesource 130, while still maintaining sufficient internal vaporconductance along the interior of the manifold to avoid an excessivevapor pressure drop. These design considerations do not limit the source130 design to cylindrical designs. Other considerations such as themethod of heating may also factor into the choice of source 130geometry, such as a square or rectangular perimeter instead of acircular perimeter. Furthermore, the present invention is not limited tolow-aspect-ratio cross sections. High-aspect-ratio cross sections, withlarge perimeter-to-area ratios, may also be used.

An advantage of sideways evaporation is that debris, comprisingunutilized evaporant 114 materials condensed on the stationary internalsof the deposition system, cannot fall onto the substrate 126 surface orinto the evaporation source 130 nozzles 136. A deposition configurationwhereby the substrates 126 are situated in a vertical configuration(“sideways evaporation”) also allows the possibility of increasedsubstrate 126 temperatures, because sagging of the substrate 126 isreduced by this orientation compared with a horizontal one.

Downwards-Evaporating Single-Nozzle Source

A downwards-evaporating configuration allows the substrate to besupported across its entire width, not just at the edges as in the caseof vertical evaporation described previously. Therefore, the potentialfor deposition at higher substrate temperatures is one advantage, amongothers, in evaporating downwards from the source onto the upward-facingsurface of the substrate (“downwards evaporation”), particularly whenthe substrate is glass. This in turn allows higher temperatures sincethe corresponding softening of the glass is mitigated by the greaterarea of support.

A number of downwards-evaporating-source configurations are herebydisclosed. One embodiment of a downwards-evaporating source according tothe invention is a single nozzle source 230, depicted in FIG. 3A asviewed from the side in cross-section. It employs an open vessel orsubstantially closed chamber containing the evaporant 214 volume and avapor space 218 above it. The crucible 246 is suspended inside anexpansion chamber 250 enclosed with in a manifold body 240. The geometryof the source 230 is typically cylindrical, i.e., having a circularcross section as viewed from the top, but it may also have other shapes,including for example square or rectangular. The crucible is centeredlaterally. Thus, in the case where the effusion source has a circularcross section, the crucible forms an annulus with the inside walls ofthe manifold body.

Underneath the bottom surface of the evaporant 214 volume, adownwards-aiming nozzle 236 directs the vapor out of the manifoldtowards the substrate 226 (not shown), which is situated below thesource 230. The design of the downwards-aiming nozzle 236 may be thatdescribed in U.S. Pat. Nos. 6,982,005 and 6,562,405, but other designsknow in the art may also be used. The source 230 is typically heatedfrom the outside surface of the manifold, in most cases by a spiralwound heating element 244 but also possibly by a serpentine heatingelement or a plurality of straight, vertically-oriented heating elementsdisposed about the circumference of the source 230. Insulation 238surrounds the device.

This particular configuration offers a number of advantages inoperation. First, by heating the outer walls of the manifold, themanifold will be maintained at a higher temperature than the melt,thereby inhibiting condensation in the manifold. Second, by suspendingthe base of the crucible 246 above the effusion nozzle 236, theevaporant 214 volume will be cooled by radiant emission through theeffusion nozzle 236. This will further reduce the temperature of theevaporant 214 volume and further inhibit condensation within themanifold. In some embodiments, the evaporant 214 volume is suspendedfrom the top surface of the source 230. The mechanical members whichaccomplish the suspension of the crucible 246 may be designed to permiteither a high or low vapor flow conductance from the evaporant 214volume to the manifold. For example, as shown in FIGS. 3A and 3B, thecrucible 246 may have the shape of a tube with a closed bottom, with thetube being suspended from and attached or sealed to the manifold andhaving restriction orifices 242 through which vapor can exit thecrucible 246 and enter the manifold interior.

The restriction orifice serves to reduce the vapor concentration (andthus, the saturation temperature) of evaporant in the manifold and alsoultimately at the nozzle. Proper sizing of the restriction orificeprevents condensation of evaporant on or In the nozzle, which is suffersconsiderable radiative heat loss and is therefore typically the cooleststructure in contact with the evaporant vapor. Restriction orifices mayalso be used in any other embodiment of the invention. FIG. 3B shows avariation on the design shown in FIG. 3A, including a crucible lid 243that incorporates one or more additional restriction orifices 245 toprovide an added expansion space 219 that affords an added increment ofvapor concentration control. Only one such additional restrictionorifices is shown in lid 243 in FIG. 3B, but any number may be used.Although FIGS. 3A and 3B depict crucible 246 with a flat bottom, theshape may differ from that and may for example be tapered downward so asto accommodate a larger volume of evaporant 214.

To achieve wide-area deposition, plural single-nozzle 236 source 230 sdescribed above may be disposed across the width of the substrate 226.The control of an individual source 230 in the cross-substrate 226 arraymay be achieved by disposing a number of composition sensing pointsequivalent to or greater than the number of single-nozzle 236 source 230s across the substrate 226, and then; 1) mathematically deriving theeffusion rates of the individual source 230 s from the compositionsensor measurements, and 2) modifying power to the individual heatingelements of the source 230 s accordingly to achieve the desiredcomposition across the width of the substrate 226.

The physical dimensions of effusion source 230 can vary according to theparticular needs of a given application. In some embodiments, theoverall diameter of the source may be in a range of about 10-14 Inches(about 25-36 cm) and the height may be in a range of about 11-16 Inches(about 28-41 cm). The nozzle 236 may typically be in a range of about2-6 cm, and more typically about 4 cm. Typical dimensions for thecrucible 246 are about 14 cm outside diameter, 12 cm inside diameter,and a height sufficient to contain a pool of evaporant about 15 cm high.There may be any number of restriction orifices 242 spaced around thecircumference of the crucible. Typically, the number is at least threeand at most twenty, for example eight such orifices each with a diameterof 2 cm. The inside diameter of manifold body 240 is typically set toprovide about a 2 cm annulus gap with the crucible.

Downwards-Evaporating Multiple-Nozzle Source

An alternate means of achieving wide-area deposition by downwardsevaporation is by a multiple-nozzle source comprising an crucible 346disposed to the side of an expansion chamber. FIG. 4 shows across-sectional side view of a multiple-nozzle, downwards-evaporatingsource 330 of this sort, according to the invention. Multiple nozzles336 (only one is visible in FIG. 4 ) are disposed along the axis normalto the plane of the page, all typically exiting from a single expansionchamber 350. The source is rectangular in cross-section when viewed fromabove, of unitary construction and formed from a single block ofmaterial. In use, translation of the substrate (not shown) Isright-to-left or left-to-right.

One or more restriction orifices 342 through the wall separating thecrucible 346 from the expansion chamber 350 allow vaporized evaporant314 to flow from the crucible into the expansion chamber. The manifoldbody 340 that surrounds the expansion chamber is contiguous with thecrucible 346. Insulation 338 surrounds the device. The restrictionorifice(s) may be of any shape, for example circular or in the shape ofa long slot running in a direction from front to back as viewed in FIG.4 . The flowing vapor is directed from the crucible 346 downwards to thesubstrate (not shown) by way of the effusion nozzles 336, for exampleaccording to the nozzle design disclosed in U.S. Pat. Nos. 6,982,005 and6,562,405. By appropriately sizing the restriction orifice(s) 342, thepressure of the vapor within the expansion chamber 350 may be reduced toinhibit condensation within the expansion chamber. A further advantageof the configuration shown in FIG. 4 is that temperature-Induced vaporpressure variations above the surface of the melt that would otherwisecause effusion non-uniformities in source configurations such as thatshown in FIG. 1 , may be mitigated by lateral vapor flow within theexpansion chamber 350.

The source 330 is heated typically by a plurality of heating elements344 oriented along the major axis (i.e. across the substrate 326 width),and located on either side of the source 330. It is desirable tominimize thermal gradients between the top and bottom of the source 330.If the bottom of the source 330 becomes too hot in relation to the topof the source 330, boiling or spitting of the evaporant 314 may occur.If the bottom of the source 330 becomes too cool in relation to the top,condensation about the effusion nozzle 336 may result in condensedevaporant 314 falling onto the substrate through nozzle 336. Suitableadjustment of the heating elements should therefore be made to maintainsufficient temperature uniformity. Access to the interior of source 330for purposes of charging evaporant to the crucible may be by one or morethreaded ports and plugs (not shown) on the upper surface of the source,and this may similarly be used for any embodiment of the invention.

Methods of Construction

The manifolds and crucibles of evaporation sources according to theinvention are typically constructed from graphite or boron nitride,although other materials may be used. Materials of construction shouldbe impervious to and non-reactive with the evaporant material at thetemperatures of use, and should remain solid and structurally strong atsuch temperatures. Temperatures are typically in a range from 1000 to1600° C.; however, the use of high vapor pressure evaporants such asselenium, which would be expected to evaporate at temperature between300-600° C., is not precluded.

To avoid vapor leakage through joints between disparate elements fromwhich the source is constructed, all joints typically will be threaded,with flat mating surfaces to provide a good seal. Optionally, joints maybe fabricated with either knife edge or flush mating surfaces andutilize a high temperature gasket material, for example a graphite foilsold under the trade name GRAFOIL® by GrafTech International of Parma,Ohio.

In the case of the sideways-evaporating source, a threaded joint may beutilized to join the manifold and the crucible in a semi-permanentfashion. Additionally, it may be desirable to incorporate a threadedplug at the top of the manifold so that the evaporant may be droppedinto the source for replenishing without substantially disassembling thesource.

In the case of the single-nozzle downwards-evaporating source, thecrucible should drop down into the top of the manifold and screw intoplace so that it is fixed in a semi-permanent fashion. Additionally, athreaded plug should be incorporated into the top surface of thecrucible for adding evaporant to the source.

In the case of the multiple-nozzle downwards-evaporating source,fabrication requires the drilling of multiple internal holes in a solidbillet of material. The crucible and expansion chamber may be fabricatedby drilling through the entirety of the length of the billet. The endsof these chambers may then be closed off by threading and installingpermanent plugs. Likewise, forming the plurality of pathways between thecrucible and the expansion chamber requires first drilling through anexternal wall of the source (either the evaporant or expansion sides) toaccess the surface of the internal separating wall, continuing onthrough the internal separating wall, removing the drill, and thenthreading and plugging the resultant holes in the external wall of thesource in a permanent fashion. Finally, a removable threaded plug orplugs should be incorporated in the top surface above the crucible forreplenishing the evaporant.

The heating elements may be a refractory metal, preferably tantalum, orless desirably tungsten, or graphite. Considerations such as the form(spiral wound, serpentine, etc.) and difficulty of fabrication assist indetermining whether a refractory metal or graphite are preferably used.

The insulation may be either a thick rigid sheet of a low-densityceramic material, an example being alumina-based foam insulation sold byZircar ceramics. Alternately, insulation may comprise a plurality ofradiation shields. Radiation shields may comprise a metal foil, graphitefoil, or thin ceramic sheet.

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimswithout departing from the invention.

We claim:
 1. A thermal evaporation source comprising: a manifold bodyincluding an expansion chamber, a nozzle, and a general sloping shapenarrowing from the expansion chamber toward the nozzle; a cruciblehaving a circumference configured to contain a volume of evaporant andsuspended from a top internal surface of the manifold body within theexpansion chamber above the nozzle, wherein mechanical members suspendthe crucible from the top internal surface and thereby form at least onerestriction orifice, the expansion chamber is a space between thecrucible and the manifold body; and wherein the at least one restrictionorifice comprises at least three orifices evenly spaced around thecircumference of the crucible, each of the at least three orifices areat a same distance from the top internal surface of the manifold.
 2. Thethermal evaporation source of claim 1, wherein the expansion chamber iscircular in cross-section.
 3. The thermal evaporation source of claim 1,further comprising a heater capable of heating the thermal evaporationsource to a temperature of between 300° C. and 1,600° C.
 4. The thermalevaporation source of claim 1, wherein the crucible is a formed of boronnitride.
 5. The thermal evaporation source of claim 1, wherein thecrucible is a formed of graphite.
 6. The thermal evaporation source ofclaim 1, wherein the at least one restriction orifice comprises eightorifices evenly spaced around the circumference of the crucible.
 7. Thethermal evaporation source of claim 1, wherein the manifold body isformed of graphite.
 8. The thermal evaporation source of claim 1,wherein the crucible includes a tapered bottom.
 9. The thermalevaporation source of claim 8, wherein the tapered bottom slopes towardthe nozzle.
 10. The thermal evaporation source of claim 1, wherein a gapexists between the crucible and the manifold body.
 11. The thermalevaporation source of claim 10, wherein the gap is at least about 2 cm.12. The thermal evaporation source of claim 1, wherein the nozzle has adiameter of between about 2 cm and about 6 cm.
 13. The thermalevaporation source of claim 1, wherein the crucible has a circularcross-section.
 14. The thermal evaporation source of claim 13, whereinthe crucible has an internal diameter of about 12 cm.
 15. The thermalevaporation source of claim 1, wherein the crucible is configured tohold a pool of the evaporant of about 15 cm in depth.
 16. The thermalevaporation source of claim 1, wherein the nozzle is oriented to directa vapor flow out of the thermal evaporation source vertically downward,in one or more horizontal directions, or in one or more directionsintermediate between horizontal and vertically downward.
 17. The thermalevaporation source of claim 1, wherein the nozzle is configured toproduce a vertically downward vapor flow.